Hydrophilic stainless steel and method for manufacturing same

ABSTRACT

A method of manufacturing hydrophilic stainless steel includes polishing the surface of the stainless steel in one direction; and immersing the polished stainless steel in an acidic solution to form an oxide film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending PCT International Application No. PCT/KR2020/006522, which was filed on May 19, 2020, and which claims priority from Korean Patent Application No. 10-2020-0007363 filed on Jan. 20, 2020. The entire contents of the aforementioned patent applications are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to hydrophilic stainless steel and a method of manufacturing the same.

BACKGROUND ART

Stainless steel is a rich resource and has strong corrosion resistance, thereby being used in various living and industrial fields such as tableware, medical appliances, power generation, aviation, construction materials, and storage tanks.

In particular, stainless steel is widely used as a material for heat exchangers in fields where polluted water, chemicals, groundwater, etc. are used because it is inexpensive among metal materials and has high corrosion resistance.

In such a heat exchanger, corrosion resistance can be secured by using stainless steel, but when used for a long time, scale is deposited on the surface of the heat exchanger.

The deposited scale has low thermal conductivity, which causes thermal insulation problems in the heat exchanger, thereby reducing the heat transfer efficiency of the heat exchanger.

As a solution to remove scale on the heat exchanger surface, the heat exchanger is cleaned periodically. However, this method also has a disadvantage in that the mechanical system must be periodically stopped, and manpower, time, and cost for cleaning are consumed.

To compensate for these shortcomings, a method for making the surface of stainless steel hydrophilic has been proposed.

A hydrophilic surface refers to a surface where a contact angle when a droplet contacts the surface is significantly lower than those of other surfaces, and water spreads.

To date, a method of performing a hydrophilic coating using an organic/inorganic material has been proposed as a method of manufacturing stainless steel having a hydrophilic surface, but water repellency cannot be exhibited without coating. In addition, the properties of the coating material are easily changed depending on external conditions, which cause a durability problem. Further, there is a risk of easily aging when used for a long period of time.

As another method, there is a method of forming a microstructure on a stainless steel surface using a laser device.

By this method, a hydrophilic stainless steel surface can be formed only by a heat treatment process without a coating process. However, since expensive equipment is used, the manufacturing cost is high, which makes practical application difficult. In addition, there is a disadvantage in that it is difficult to increase an area for mass production.

DISCLOSURE Technical Problem

Therefore, the present disclosure has been made in view of the above problems, and it is one object of the present disclosure to provide hydrophilic stainless steel whose surface has unidirectionality by physically polishing the surface in one direction, thereby having hydrophilicity; and a method of manufacturing the same.

It is another object of the present disclosure to provide hydrophilic stainless steel whose surface is physically polished, and then thermally treated at high temperature, so that the stainless steel surface has a micro-sized surface structure and nano-sized surface structure thereon, thus having hydrophilicity; and a method of manufacturing the same.

It is still another object of the present disclosure to provide hydrophilic stainless steel manufactured in a method of thermally treating at high temperature after physical polishing, so that the manufacturing costs of the stainless steel are low compared to an existing method of making the surface of stainless steel hydrophilic in a coating manner; and a method of manufacturing the same.

It is still another object of the present disclosure to provide hydrophilic stainless steel whose surface is thermally treated after physical polishing, thereby improved durability; and a method of manufacturing the same.

It is yet another object of the present disclosure to provide hydrophilic stainless steel whose hydrophilic properties can be imparted only by physically polishing and high-temperature heat treatment, thus allowing a large-area process; and a method of manufacturing the same.

Technical Solution

In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by the provision of a method of manufacturing hydrophilic stainless steel, the method including: polishing the surface of the stainless steel in one direction; and immersing the polished stainless steel in an acidic solution to form an oxide film.

According to an embodiment of the present disclosure, the polished stainless steel may have a micro-sized surface structure.

According to an embodiment of the present disclosure, the method may include, after the immersing of the polished stainless steel, raising the temperature of the stainless steel on which the oxide film has been formed, and then thermally treating the stainless steel; and cooling the thermally treated stainless steel.

According to an embodiment of the present disclosure, the thermally treated stainless steel may have a nano-sized surface structure.

According to an embodiment of the present disclosure, the thermally treating may be performed at 850° C. to 1150° C.

According to an embodiment of the present disclosure, the thermally treating may be performed for 10 minutes to 60 minutes.

According to an embodiment of the present disclosure, in the raising of the temperature of the stainless steel, a temperature of from 550° C. to 850° C. may be achieved by raising at a temperature raising rate of 200° C./sec to 300° C./sec.

According to an embodiment of the present disclosure, in the cooling of the thermally treated stainless steel, cooling from 850° C. to 550° C. may be performed at a cooling rate of 200° C./sec to 300° C./sec.

In accordance with another aspect of the present disclosure, there is provided hydrophilic stainless steel manufactured according to the method of manufacturing hydrophilic stainless steel of the present disclosure.

According to an embodiment of the present disclosure, the hydrophilic stainless steel may have a contact angle of 0° to 30°.

Advantageous Effects

According to an embodiment of the present disclosure, the surface of stainless steel can be imparted with hydrophilicity by physically polishing the stainless steel surface in one direction to form a micro-sized surface structure having unidirectionality thereon.

According to another embodiment of the present disclosure, the surface of stainless steel can be imparted with hydrophilicity by physically polishing the stainless steel surface, and then thermally treating the surface at high temperature to form a micro-sized surface structure and nano-sized surface structure thereof.

According to still another embodiment of the present disclosure, manufacturing costs can be reduced, compared to an existing method of making the surface of stainless steel hydrophilic in a coating manner, by using a method of physically polishing stainless steel, and then thermally treating the stainless steel at high temperature.

According to still another embodiment of the present disclosure, the durability of stainless steel can be improved by physically polishing the surface of stainless steel, and then thermally treating the surface at high temperature.

According to yet another embodiment of the present disclosure, the surface of stainless steel can be imparted with hydrophilic properties only through physical polishing and high-temperature heat treatment, which allows a large-area process.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a flowchart of a method of manufacturing hydrophilic stainless steel according to an embodiment of the present disclosure.

FIG. 2 illustrates a scanning electron microscopy (SEM) image of the stainless steel surface of Example 1.

FIG. 3 illustrates an SEM image of the stainless steel surface of Example 2.

FIG. 4 is an image illustrating a measurement result of a contact angle with respect to the stainless steel of Example 1 using a contact angle measuring instrument.

FIG. 5 is an image illustrating a result of measuring a contact angle with respect to the stainless steel of Example 2 using a contact angle measuring instrument.

FIG. 6 illustrates a scanning electron microscopy (SEM) image of the stainless steel surface of Comparative Example 1.

FIG. 7 illustrates a result of measuring a contact angle with respect to the stainless steel of Comparative Example 1 using a contact angle measuring instrument.

BEST MODE

The present disclosure will now be described more fully with reference to the accompanying drawings and contents disclosed in the drawings. However, the present disclosure should not be construed as limited to the exemplary embodiments described herein.

The terms used in the present specification are used to explain a specific exemplary embodiment and not to limit the present inventive concept. Thus, the expression of singularity in the present specification includes the expression of plurality unless clearly specified otherwise in context.

It will be further understood that the terms “comprise” and/or “comprising”, when used in this specification, specify the presence of stated components, steps, operations, and/or elements, but do not preclude the presence or addition of one or more other components, steps, operations, and/or elements thereof.

It should not be understood that arbitrary aspects or designs disclosed in “embodiments”, “examples”, “aspects”, etc. used in the specification are more satisfactory or advantageous than other aspects or designs.

In addition, the expression “or” means “inclusive or” rather than “exclusive or”. That is, unless otherwise mentioned or clearly inferred from context, the expression “x uses a or b” means any one of natural inclusive permutations.

In addition, as used in the description of the disclosure and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless context clearly indicates otherwise.

Although terms used in the specification are selected from terms generally used in related technical fields, other terms may be used according to technical development and/or due to change, practices, priorities of technicians, etc. Therefore, it should not be understood that terms used below limit the technical spirit of the present disclosure, and it should be understood that the terms are exemplified to describe embodiments of the present disclosure.

Also, some of the terms used herein may be arbitrarily chosen by the present applicant. In this case, these terms are defined in detail below. Accordingly, the specific terms used herein should be understood based on the unique meanings thereof and the whole context of the present disclosure.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Meanwhile, in the following description of the present disclosure, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure unclear. In addition, the terms used in the specification are defined in consideration of functions used in the present disclosure, and can be changed according to the intent or conventionally used methods of clients, operators, and users. Accordingly, definitions of the terms should be understood on the basis of the entire description of the present specification.

FIG. 1 illustrates a flowchart of a method of manufacturing hydrophilic stainless steel according to an embodiment of the present disclosure.

The method of manufacturing hydrophilic stainless steel according to an embodiment of the present disclosure (hereinafter referred to as “the method of the present disclosure”) includes the step (S110) of polishing the surface of the stainless steel in one direction and a step (S120) of immersing the polished stainless steel in an acidic solution to form an oxide film.

In S110, the surface of the stainless steel may be physically polished in one direction using a grinding stone.

The grinding stone may be a grinding stone of 40 to 200 grit. To increase the surface roughness of the stainless steel, the surface of the stainless steel may be physically polished using a grinding stone of preferably 40 to 100 grit.

The physically polishing is a basic operation for making the surface of the stainless steel hydrophilic.

In S110, it is preferable to physically polish the surface of the stainless steel in one direction so that water on the surface of the stainless steel is drained and the water rapidly spreads on the stainless steel surface.

The physically polished stainless steel may have a micro-sized surface structure, specifically a micro-sized concave-convex structure having unidirectionality.

According to an embodiment, the surface of the stainless steel may be washed with water before or after S110 to remove residues present on the stainless steel surface.

In S120, an oxide film may be formed the surface of the physically polished stainless steel by immersing the physically polished stainless steel in an acidic solution.

In S110, an oxide film is formed on the surface of the stainless steel because the surface is exposed to air, but the oxide film may be removed by physical polishing.

In S120, an oxide film may be formed again on the stainless steel surface whose oxide film has been removed by physical polishing.

The acidic solution may excessively supply oxygen to the physically polished stainless steel, thereby forming an oxide film including chromium (Cr) on the physically polished stainless steel surface.

The acidic solution may be, for example, a nitric acid (HNO₃) solution, but is not limited thereto.

For example, in S120, an oxide film may be formed on the polished stainless steel surface by immersing the polished stainless steel in a 30% nitric acid solution at 50° C. for 100 minutes.

After S120 of the method of the present disclosure, a step (S130) of raising the temperature of the stainless steel on which the oxide film is formed, and then thermally treating the stainless steel; and a step (S140) of cooling the thermally treated stainless steel may be included.

In S130, the temperature of the stainless steel on which the oxide film is formed by S120 may be rapidly raised to a heat treatment process temperature under a general atmosphere or oxygen atmosphere, and then may be thermally treated during a heat treatment process time to further improve the hydrophilicity of the stainless steel surface.

At this time, the heat treatment process temperature means a target temperature to be reached by raising the temperature of the stainless steel having the oxide film formed thereon, and the time (hereinafter referred to as “heat treatment process time”) during which the heat treatment process is performed means a time during which the heat treatment process temperature is maintained after the stainless steel having the oxide film formed thereon reaches the heat treatment process temperature.

In S130, chromium (Cr) included in the stainless steel on which the oxide film is formed through the heat treatment may be formed into nano-sized crystals.

That is, the stainless steel thermally treated through S130 may have a nano-sized surface structure. Specifically, the thermally treated stainless steel surface may have nano-sized chromium oxide crystals.

Accordingly, the thermally treated stainless steel surface may have a nano-sized surface structure formed through the thermal treatment together with a micro-sized surface structure formed through the physical polishing.

The nano-sized surface structure formed on the thermally treated stainless steel surface may improve the hydrophilic properties of the stainless steel surface. Cr₂O₃ is a ceramic material with high surface energy, and even a simple Cr₂O₃ layer with no structure has a low contact angle (CA). In addition, when a nanostructure is formed on the stainless steel surface, hydrophilicity is improved as a hydrophilic area is widened. Therefore, when a nano-sized Cr₂O₃ structure is formed on the surface, super-hydrophilic properties may be expressed and improved.

In the case of an existing technology, to eliminate defects on the stainless steel surface and improve hydrophilic properties thereof, physical polishing is performed using a grinding stone having low roughness, or heat treatment is performed at high temperature for a long time.

However, the present disclosure may improve the hydrophilic properties of the stainless steel surface rather by utilizing defects (i.e., a micro-sized surface structure and a nano-sized surface structure) formed on the stainless steel surface.

Accordingly, the present disclosure may improve the hydrophilic properties of the stainless steel surface by forming a nano-sized surface structure on the stainless steel surface through heat treatment at a lower temperature than in the existing technology.

Specifically, the heat treatment process temperature may be 850° C. to 1150° C. When the heat treatment process temperature is less than 850° C., a nano-sized surface structure may not formed on the stainless steel surface on which the oxide film has been formed.

When the heat treatment process temperature exceeds 1150° C., an austenite phase used in the present disclosure may be deformed, which may cause a problem in that physical and chemical properties of the material are changed.

The heat treatment process time may be 10 to 60 minutes. When the heat treatment process time is less than 10 minutes, a nano-sized surface structure may not formed on the stainless steel surface through the heat treatment.

In S130, since different phases may be formed and corrode on the surface of the stainless steel at 700° C., it is preferable to rapidly raise a temperature such that the temperature of the stainless steel on which the oxide film is formed reaches the heat treatment process temperature.

According to an embodiment, the temperature raising process of S130 may include a first temperature raising process; and a second temperature raising process whose temperature raising rate is faster than that of the first temperature raising process.

For example, in S130, the temperature of the stainless steel on which the oxide film has been formed is raised at temperature raising rate of 100° C./min through the first temperature raising process, and then 550° C. may reach the heat treatment process temperature, i.e., 850° C., by raising temperature at a temperature raising rate of 200° C./sec or more through the second temperature raising process. That is, the temperature of the second temperature raising process should be rapidly raised at a rate of 200° C. or more per second.

Specifically, the temperature of the stainless steel on which the oxide film has been formed may be rapidly raised at a temperature raising rate of 200° C./sec to 300° C./sec to prevent corrosion of the stainless steel during the temperature raising process.

That is, when out of the temperature raising rate range and the heat treatment temperature range, carbide precipitation occurs at a metal grain boundary. In the case of carbide, since carbide is a highly corrosive material compared to stainless steel, corrosion easily occurs depending on a carbide precipitated at a grain boundary, and the metal grain boundary itself separates and falls off.

In S140, stainless steel having a hydrophilic surface may be manufactured by cooling the thermally treated stainless steel.

The thermally treated stainless steel may be cooled from 850° C. to 550° C. at a cooling rate of 200° C./sec to 300° C./sec. Since other phases may be formed and corroded on the surface of the stainless steel at 700° C. as in the second temperature raising step, cooling is rapidly performed from 850° C. to 550° C.

The hydrophilic stainless steel manufactured by the method of the present disclosure may have a surface contact angle of 0° to 30°.

Specifically, the contact angle of the physically polished stainless steel may be 10° to 30°, and the contact angle of the stainless steel thermally treated after the physically polishing may be 0° to 10°.

According to an embodiment, after S140, the cooled stainless steel may be washed to remove residues remaining on the surface of the cooled stainless steel.

According to the present disclosure, stainless steel having super-hydrophilic surface properties may be manufactured by thermally treating the stainless steel surface at a high temperature after physical polishing.

In addition, the present disclosure uses a method of thermally treating at a high temperature after physical polishing, so that the manufacturing cost may be reduced compared to a method for manufacturing a hydrophilic surface of stainless steel through a conventional coating method.

Further, by thermally treating the stainless steel surface at a high temperature after physical polishing according to the present disclosure, durability may be increased and a large area may be manufactured.

Hereinafter, the surface of stainless steel was treated through the examples, and then the stainless steel surface was observed and the contact angle (CA) thereof was measured.

EXAMPLE 1

After washing the surface of stainless steel, the stainless steel surface was polished in one direction using a 60# grinding stone to form a micro-sized surface structure.

Next, the surface was washed with water to remove residues remaining on the polished stainless steel surface.

After washing, the washed stainless steel was immersed in a 30% nitric acid solution at 70° C. for 60 minutes to re-form an oxide film removed from the stainless steel surface, thereby forming an oxide film.

EXAMPLE 2

After washing the surface of stainless steel, the stainless steel surface was polished in one direction using a 60# grinding stone to form a micro-sized surface structure.

Next, the surface was washed with water to remove residues remaining on the polished stainless steel surface.

After washing, the washed stainless steel was immersed in a 30% nitric acid solution at 70° C. for 60 minutes to re-form an oxide film removed from the stainless steel surface, thereby forming an oxide film.

Next, to form a nano-sized surface structure on the stainless steel surface, the temperature of the stainless steel was raised to 850° C. under a general atmosphere, and then thermally treated for 30 minutes.

After cooling the thermally treated stainless steel, the stainless steel was washed to remove residues remaining on the stainless steel surface.

Comparative Example 1

After washing the surface of stainless steel surface, the stainless steel surface was not polished in one direction, the temperature of the stainless steel was raised to 850° C. at 100° C/min, and heat treatment was not performed.

PROPERTY EVALUATION

FIG. 2 illustrates a scanning electron microscopy (SEM) image of the stainless steel surface of Example 1.

Referring to FIG. 2, it can be seen that a micro-sized concave-convex structure formed in one direction is formed on the stainless steel surface of Example 1.

That is, due to the micro-surface structure formed in one direction, water on the stainless steel surface may be drained and spread along a direction in which the micro-surface structure is formed, so that the stainless steel surface have hydrophilicity.

FIG. 3 illustrates an SEM image of the stainless steel surface of Example 2. Referring to FIG. 3, it can be seen that nano-sized chromium crystals are formed together with a micro-sized concave-convex structure having directionality on the stainless steel surface of Example 2.

Therefore, nano-sized chromium crystals are formed through high-temperature heat treatment in addition to the micro-surface structure according to the present disclosure, so that hydrophilic properties are improved.

FIG. 4 is an image illustrating a measurement result of a contact angle with respect to the stainless steel of Example 1 using a contact angle measuring instrument.

Referring to FIG. 4, it can be seen that the contact angle of Example 1 is 16°. That is, it can be confirmed that the present disclosure has hydrophilicity by forming a micro-sized surface structure having unidirectionality on the stainless steel surface through physical polishing.

FIG. 5 is an image illustrating a result of measuring a contact angle with respect to the stainless steel of Example 2 using a contact angle measuring instrument.

Referring to FIG. 5, it can be seen that the contact angle of Example 2 is almost 0°. Through this, it can be confirmed that the nano-sized surface structure formed through high-temperature heat treatment is formed, together with the micro-sized surface structure formed by physical polishing, on the stainless steel surface according to the present disclosure, so that hydrophilic properties are further improved.

FIG. 6 illustrates a scanning electron microscopy (SEM) image of the stainless steel surface of Comparative Example 1.

Referring to FIG. 6, it can be seen that a micro-sized concave-convex structure with directionality and nano-sized chromium crystals are not formed on the stainless steel surface.

FIG. 7 illustrates a result of measuring a contact angle with respect to the stainless steel of Comparative Example 1 using a contact angle measuring instrument.

Referring to FIG. 7, it can be seen that the contact angle of Comparative Example 1 is very large as 34°.

Therefore, it can be confirmed that, unlike Comparative Example 1 that has not been subjected to polishing in one direction and heat treatment, the present disclosure forms a nano-sized surface structure formed through high-temperature heat treatment, together with the micro-sized surface structure formed by physical polishing, on the stainless steel surface, so that hydrophilic properties are further improved.

Although the present disclosure has been described through limited examples and figures, the present disclosure is not intended to be limited to the examples. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the disclosure. Therefore, it should be understood that there is no intent to limit the disclosure to the embodiments disclosed, rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the claims. 

What is claimed is:
 1. A method of manufacturing hydrophilic stainless steel, the method comprising: polishing the surface of the stainless steel in one direction; and immersing the polished stainless steel in an acidic solution to form an oxide film.
 2. The method according to claim 1, wherein the polished stainless steel has a micro-sized surface structure.
 3. The method according to claim 1, comprising: after the immersing of the polished stainless steel, raising a temperature of the stainless steel on which the oxide film has been formed, and then thermally treating the stainless steel; and cooling the thermally treated stainless steel.
 4. The method according to claim 3, wherein the thermally treated stainless steel has a nano-sized surface structure.
 5. The method according to claim 3, wherein the thermally treating is performed at 850° C. to 1150° C.
 6. The method according to claim 3, wherein the thermally treating is performed for 10 minutes to 60 minutes.
 7. The method according to claim 3, wherein, in the raising of the temperature of the stainless steel, a temperature of from 550° C. to 850° C. is achieved by raising at a temperature raising rate of 200° C./sec to 300° C./sec.
 8. The method according to claim 3, wherein, in the cooling of the thermally treated stainless steel, cooling from 850° C. to 550° C. is performed at a cooling rate of 200° C./sec to 300° C./sec.
 9. Hydrophilic stainless steel manufactured according to the method of claim
 1. 10. The hydrophilic stainless steel according to claim 9, wherein the hydrophilic stainless steel has a contact angle of 0° to 30°. 